1. Field of the Invention
The present invention relates to an apparatus and a method for receiving control channels in a mobile communication system providing packet data services. More particularly, the present invention relates to an apparatus and a method for improving reception reliability for control channels by detecting time division multiple (TDM) frames transmitted through the control channels in a TDM mobile communication system.
2. Description of the Related Art
Mobile communication systems are now providing users with data communication services in addition to voice communication services. Such mobile communication systems can provide not only various data services, but also various broadcast services. To support the broadcast services, broadcast data are transmitted using a time division multiple (TDM) scheme in a frame unit. If the broadcast data are transmitted through the TDM scheme, receivers can selectively receive a minimum amount of frames so that the reception efficiency of the data can be improved.
Various standardizations have been prepared in the 3GPP2 group to provide the broadcast services by using a code division multiple access (CDMA) scheme. According to the CDMA 2000 1×Rev. D standardization, which has been suggested by the 3GPP2 group, the broadcast service is called a “broadcast multicast service”. The CDMA 2000 1×Rev. D standardization defines the size of physical channels used in a 1×EV-DV (Evolution for Data & Voice) system capable of simultaneously providing packet services and voice services. Hereinafter, the description will be made in relation to a forward dedicated channel (F-DCCH) of the physical channels, which is a control channel for providing users with control information in a Broadcast and Multicast Service (BCMCS) service.
According to the BCMCS service, as opposed to a unicast scheme, broadcast data or push to talk (PTT) service data are transmitted to a plurality of users through one physical channel. The control information is transmitted to the users through the F-DCCH in a forward link of the 1×EV-DV system providing the BCMCS service. In addition, the users may share the same physical channel through the TDM scheme and the users are differentiated from each other by means of a long code mask.
FIG. 1 is a block diagram illustrating the structure of a transmitter 100 and a receiver 200 in a conventional mobile communication system. Hereinafter, the standardization for a baseband physical channel of a F-DCCH used as a control channel in the 1×EV-DV system and the structure of the receiver 200 will be described with reference to FIG. 1.
Referring to FIG. 1, a cyclic redundancy checking (CRC) inserter 110 inserts CRC bits into information bits in order to check for errors in the information bits transmitted to specific users as control information and sends the information bits with the CRC bits to a channel coder 120. The channel coder 120 codes the information bits having the CRC bits through a predetermined channel coding scheme and sends the channel-coded information bits to a scrambler 130. The channel coding scheme may include a convolutional coding scheme.
The scrambler 130 performs long code mask scrambling with respect to channel-coded information bits to provide security to the channel-coded information bits per each user. Then, the scrambler 130 sends the channel-coded information bits to a spreader 140. Upon receiving the channel-coded information bits, the spreader 140 spreads the channel-coded information bits by using PN codes and Walsh codes and sends them to a radio network. Thus, the spread information bits in the form of frame units are transmitted to the receiver 200 through the F-DCCH.
As the spread information bits have been received in the receiver 200, a despreader 210 of the receiver 200 despreads the information bits of the frames by using the PN codes and Walsh codes and sends them to a descrambler 220. Thus, the descrambler 220 descrambles the despread information bits by using the long code mask and send them to a channel decoder 230. Upon receiving the descrambled information bits, the channel decoder 230 decodes the descrambled information bits and sends the decoded information bits to a CRC inspector 240. If a convolution coder is used as the channel coder 120, a Viterbi decoder is used as the channel decoder 230.
The CRC inspector 240 checks for a transmission error in the frame by using the CRC bits contained in the decoded bits and outputs error detection bits for determining the validity of the frame, that is, decoded bits.
According to the 1×EV-DV system, the users may share the same F-DCCH channel, through which information bits in the form of frame units are transmitted as control information for supporting the broadcast service, by means of the TDM scheme. In addition, the users may be differentiated from each other by using the long code mask.
FIG. 2 is a schematic diagram illustrating the structure of a control channel through which control information is transmitted using the TDM scheme in the conventional mobile communication system. The control channel shown in FIG. 2 is the F-DCCH and control information transmitted to users (user 1, user 2, user 3, etc) is time-multiplexed in a frame unit (TF). In addition, the control information, that is, the information bits are coded by using the long code mask allocated to each user, thereby differentiating the control information transmitted to each user.
The receiver 200 shown in FIG. 1 descrambles the coded information bits by using the long code allocated to each mobile station. At this time, if the descrambled information bits belong to the TDM frame allocated to the receiver 200, the CRC inspector 240 of the receiver 200 outputs an error detection bit “Good” based on the characteristic of a code word. However, if the descrambled information bits belong to the TDM frame allocated to the other receiver, the CRC inspector 240 of the receiver 200 outputs an error detection bit “Bad” due to the discord of the long code. In addition, the CRC inspector 240 of the receiver 200 outputs the error detection bit “Bad” when the reception quality of the frame is lowered so that the receiver 200 may detect whether the reception frame is adaptable for the receiver 200.
In this manner, each user can check whether the frame received through the control channel, that is, the F-DCCH is adaptable for the user by using the long code and the CRC. However, if the users of the frame are differentiated from each other through the above manner, it is difficult to ensure the reception reliability of the control channel.
That is, when the frame having a length of 20 ms is transmitted through the F-DCCH according to the CDMA 2000 1×Rev. D standardization, a 12-bit CRC is used to detect the frame error. In this case, the frames transmitted to other users may not match with the code word because scrambling and descrambling processes for the frames are performed based on mutually different long codes after the frames have been descrambled in the receiver 200, so that the frames represent abnormal characteristics as if they have no control signals. Accordingly, if reception signals for the frames allocated to other users or the frames having no control information are output from the channel decoder 230 in the form of a predetermined sequence, a false alarm probability (FAR) of the CRC is about 2.44×10−4, which means that the detection error periodically occurs in every 82 seconds.
Therefore, if the service is provided for a long period of time, such as the broadcast service or the PTT service, the FAR of the CRC is very high. In addition, since the F-DCCH transmits an L3 signaling signal (that is, the control information), it is necessary to ensure superior reception reliability with respect to the transmission signal of the F-DCCH in order to stably provide the users with broadcast services.
Therefore, a need exists for an apparatus and method for reducing the amount of errors in a broadcast service.